Pass argc pointer to parse_options()

This lets us keep argc up to date as the backend picks out arguments
from the argv array.
diff --git a/clients/keyboard.c b/clients/keyboard.c
index 3760ff7..ad34b85 100644
--- a/clients/keyboard.c
+++ b/clients/keyboard.c
@@ -706,7 +706,7 @@
 
 	memset(&virtual_keyboard, 0, sizeof virtual_keyboard);
 
-	virtual_keyboard.display = display_create(argc, argv);
+	virtual_keyboard.display = display_create(&argc, argv);
 	if (virtual_keyboard.display == NULL) {
 		fprintf(stderr, "failed to create display: %m\n");
 		return -1;